There are many types of electrical switches that have been used in the past for switching electrical current. In its most simplest form, a switch has two electrically conductive contacts that touch each other to allow electrical current to flow through a circuit, “making” the circuit, and that are separated when it is desired to prevent current from flowing through the circuit, “breaking” the circuit. Another type of switch commonly used is a semiconductor switch, which is of non-mechanical construction and typically employs one or more transistors or the like.
Some types of mechanical switches are manually actuated. Examples of manually actuated switches include, for example, light switches, push-button switches, toggle switches, rocker switches, and rotary switches. These switches are manually actuated because they require a person to manually engage them, such as by using one or more fingers to press, turn or otherwise move the switch to a desired position.
Other types of mechanical switches employ an actuator that is used to move a switch to a desired position, such as to turn it on or off. A switch actuator is a device that transforms some sort of an input into motion that moves the switch to the desired position. Operation of a switch actuator is typically controlled in some sort of automated manner. For example, a sensor, such as a voltage sensor, a current sensor, a temperature sensor or another kind of a sensor or sensor arrangement can be used to provide an input to the actuator that causes it to move a switch to a desired position.
Not only can such switches be used to turn power on or off, they can also be employed to switch between two or more different inputs or sources, including, among other things, power sources. A switch used to switch between two different inputs is called a transfer switch. Transfer switches are most commonly used to switch an electrical load between two different input power sources. Like virtually all types of switches, transfer switches can be of mechanical or semiconductor construction and can be manually actuated or automatically actuated. Automatically actuated mechanical transfer switches employ an actuator to move the transfer switch to the desired position.
Manual transfer switches are usually mechanical and are used to manually switch between two different inputs or sources. In electrical power switching applications, they are used to switch an electrical load between two or more input sources of electrical power.
One type of transfer switch, such as typically used for whole house power switching of power from utility power to generator power, is configured to provide a break-before-make or open transition switching arrangement. The term “break-before-make” means that the transfer switch first breaks the electrical connection with one source of electrical power before making the electrical connection with the other source of electrical power. For example, when switching over to generator power, the transfer switch breaks the utility power connection before making the generator connection to prevent unwanted and potentially damaging power back-feed.
A second type of transfer switch, often referred to as a “make-before-break” or closed transition transfer switch, is used in applications where it is desired to make the connection with the second input before breaking the connection with the first input. In power switching applications, a “make-before-break” power transfer switch allows a hot-to-hot transfer without loss of critical load. To put it a different way, a “make-before-break” transfer switch permits switching between active or “hot” input sources.
A third type of transfer switch is typically referred to as a center off or delayed transition transfer switch. Delayed transition transfer switches are nearly always used in applications involving large inductive loads that can undesirably cause large inrush currents. In a delayed transition transfer switch, there is an intentional time delay between the breaking of the connection with the first input before making the connection with the second input. A time delay after breaking the first connection is provided sufficient to permit magnetic fields of the inductive load to completely collapse before making the second connection.
Mechanical transfer switches can employ an electromechanical actuator, such as a solenoid actuator, to move the transfer switch to the desired position. When it is desired to change transfer switch position, an input, in the form of electrical current, is applied to the actuator, or removed from the actuator, to cause an armature of the actuator to move in a desired direction as well as in at least some instances, to a desired position. Because the armature is connected or otherwise mechanically linked to the switch, movement of the armature also causes the switch to move. Therefore, controlled application of electrical current to the actuator causes its armature to move in a desired direction, typically to a desired position, causing the switch to move along with it to the desired switching position.
FIG. 1 depicts an exemplary prior art solenoid actuator 40 like that commonly used in electromechanical transfer switches. The solenoid 40 includes a stationary stator 42 that has a fixed electrical coil 44 around a center pole 46 of the stator 42 that is energized to urge an armature 48 made of magnetic material toward it in the manner depicted in FIG. 1. Although not shown in FIG. 1, a spring is used to return the armature 48 back to where it was originally located before energization of the coil 44. Unfortunately, due at least in part to its relatively high inertia armature construction, solenoid actuators 40 have undesirably slow response times, which limits switching speeds and transfer times when used in electromechanical transfer switches.
Another commonly used electromechanical actuator used in electromechanical transfer switches is a rotary or stepper motor-type electromechanical actuator. However, these electromechanical transfer switch actuators do nothing to remedy the deficiencies found in the aforementioned solenoid actuators. In fact, these types of actuators are often part of a relatively complex transfer switch mechanism that includes cams, gears, linkages and the like. Not only are such transfer switch mechanisms unacceptably slow for more demanding transfer switch applications, their components can undesirably wear over time or even stick, if not frequently tested, resulting in premature or even unexpected failure.
Where a transfer switch is automatically controlled, it is commonly referred to as an automatic transfer switch or ATS. Electromechanical automatic transfer switches utilize an electromagnetic actuator, such as an aforementioned solenoid or rotary or stepper motor actuator. Solid state automatic transfer switches, as discussed in more detail below, utilize semiconductor switching technology and are used in transfer switching applications where fast switching is required.
While transfer switches that employ an electromagnetic actuator have enjoyed substantial commercial success, improvements nonetheless remain desirable. For example, conventional solenoid actuators and rotary stepper motor actuators in the past have been inherently slow operating, limiting their use to switching applications tolerant of their slow switching speeds and slow transfer times.
With specific regard to automatic transfer switching applications, it has been a challenge to achieve transfer times faster than about seven alternating current cycles, e.g., less than 120 milliseconds where sixty hertz alternating current is used, using a transfer switch that is electromagnetically actuated. Indeed, it has been believed heretofore unknown to achieve transfer times faster than two cycles, e.g., less than 33.3 milliseconds where sixty hertz alternating current is used, in an automatic transfer switch that is electromagnetically actuated. As a result, electromagnetically actuated automatic transfer switches have been limited to less critical switching applications, such as those where the load can tolerate up to five cycles of power loss during switching.
Where faster transfer or switching times are required, semiconductor or solid state transfer switches are used. These are used for more critical transfer switching applications, including those where the load cannot tolerate loss of power for very long during switching. Semiconductor automatic transfer switches have been commercially available for quite some time that provide sub-cycle transfer times, thereby enabling switching to be performed in 17milliseconds or less. Some semiconductor automatic transfer switches can perform switching as fast as one-half of a cycle, e.g., 8.3 milliseconds, or even faster in some instances. An automatic transfer switch capable of such high speed transfer times is referred to as a static transfer switch.
FIG. 2 is a circuit schematic that illustrates an example of a simple prior art solid state static transfer switch 50. The transfer switch 50 has two pairs of silicon controlled rectifiers (SCRs) 52 and 54 with one pair 52, also labeled Q1 and Q2, being arranged in a back-to-back configuration to enable one of the inputs 56, also labeled S1, to be connected a load 58 and the other pair 54, also labeled Q3 and Q4, being arranged back-to-back to enable the other one of the inputs 60, also labeled S2, to be connected to the load 58.
During operation, SCRs Q1 and Q2 are turned on to connect input S1 to the load 58. Where it is desired to connect the other input S2 to the load 58, such as where some aspect of input S1 is not satisfactory, SCRs Q1 and Q2 are turned off and SCRs Q3 and Q4 are turned on. When SCRs Q3 and Q4 are turned on, input S2 becomes connected to the load 58. Where input S1 represents utility power and input S2 represents a source of backup power, operation of such a static transfer switch 50 can be controlled to switch from utility power to backup power when the need arises.
Thyristors or SCRs are typically used in solid state transfer switches because they are more robust, have lower power losses, and can be used with simpler low-power control arrangements than other types of semiconductor switches. SCRs are particularly well suited for high power switching applications because of their ability to switch electrical currents ranging from a few amps up to a few thousand amps, which can amount to millions of watts in some power switching applications.
Unfortunately, SCRs have certain disadvantages. For example, SCRs dissipate more power than electromechanically actuated transfer switches sometimes producing a significant amount of heat during operation. As a result, cost and complexity is often increased as additional equipment may be needed to remove the heat produced by the SCRs. Where heat transfer equipment is needed, transfer switch maintenance and monitoring costs typically are undesirably increased.
Another drawback lies in the fact that SCRs always require a supply of control power to maintain connection between an input source and the load. Should the control power to the transfer switch fail, all of the SCRs will switch off, breaking each and every connection between input and load. To prevent this from happening, solid state transfer switches are usually equipped with redundant power supplies. Unfortunately, the level of redundancy typically required to ensure reliable and stable operation undesirably increases its purchase price, adds to complexity, and requires additional monitoring and maintenance, all of which is undesirable and adds to overall operational costs.
A still further drawback lies in the fact that an SCR cannot be turned off by simply telling it to “turn off” or by sending it a “turn off” signal. By their inherent nature, an SCR will only break the electrical connection it has made between input and load when electrical current applied to its main or power terminal falls to a sufficiently low magnitude, typically zero, such that the SCR completely turns itself off. In certain instances when trying to turn an SCR off, it can take an unacceptably long time for the current to drop low enough for the SCR to actually turn off and break the flow of power to the load. Sometimes, this delay can leave the load without power or sufficient quality power, which can adversely affect load operation. In some cases, its operation can cease and damage can occur.
This inability to turn off an SCR under electronic control in a predictable, repeatable, and consistent manner also makes it difficult to guarantee that current flow to the load from one input source will be switched off before current flow from the other input source is switched on. Should current flow from both input sources end up being provided to the load at the same time, such as what can happen if the control current does not reach a low enough value to cause the SCRs connected to the one input source to switch off in time before the other SCRs turn on connecting the load to the other input source, both input sources can end up becoming shorted together. Even though this might happen for only a relatively short period of time, this kind of short circuiting can damage the transfer switch, can adversely impact operation of the load, can adversely affect other loads connected elsewhere to one or both input sources, and can even cause both input sources to catastrophically fail, leading to complete power loss. This type of short circuiting is more commonly known as cross-conduction or shoot-though, and is a well-known failure mode of solid-state transfer switches.
To attempt to prevent this, solid state automatic transfer switches often have a great deal of built-in redundancy and typically require relatively complex control circuitry and control logic. As a result, cost is undesirably increased and their required complexity alone compromises reliability. Where used for particularly critical transfer switching applications, monitoring, testing and maintenance requirements are more stringent and costly, all of which is undesirable.
Finally, because of packaging size constraints, SCRs, as with any type of solid state switch, are simply not able to meet certain international safety standards because their terminals are inherently spaced too close together. Where these and other similarly stringent standards come into play, solid state switches typically cannot be used.
While there are other types of solid state switches, their limitations are so great that they have found, at best, only limited use in transfer switches. For example, there are types of semiconductor switches, including bipolar transistors, insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs), and others, that can be turned off via a “turn off” signal. However, they all generally suffer from unacceptably high conduction losses in high current and high voltage applications, such as what is frequently encountered in transfer switching applications. In addition, they typically are not robust enough for most, if not virtually all, static transfer switch applications. For example, these types of semiconductor switches are often unable to withstand high short-circuit currents without failing or undesirably degrading in performance.
What is needed is a switching arrangement that is versatile, robust, and capable of use in transfer switching applications, including power transfer switching applications. What is also needed is a switching arrangement that overcomes at least some of the aforementioned drawbacks and disadvantages.